1. Field of the Invention
The present invention relates to a sheet stacking apparatus in which sheets discharged from an image forming apparatus such as a copying machine, a laser beam printer and the like are stacked on a sheet stacking plate, and an image forming apparatus having such a sheet stacking apparatus, and more particularly, it relates to control of a position of such a sheet stacking plate.
2. Related Background Art
Among conventional image forming apparatuses such as copying machines, laser beam printers and the like, there is an image forming apparatus having a post-treatment apparatus for effecting a post-treatment such as sheet stapling. In such a post-treatment apparatus, as shown in FIG. 42, a tray (sheet stacking plate) 103 on which sheets are stacked is provided on a lift/lower tray shifting stage 102. On the other hand, an upper rockable guide 88 is provided with a sheet level detection sensor 105 for detecting the fact that a height of a sheet stack S comprised of sheets discharged on the tray 103 reaches a predetermined level. The sheet level detection sensor 105 includes a pivotable sensor lever 106 contacted with the sheet stack S rested on the tray 103 and having an upper end pivotablly supported, and a photo-sensor 107 for emitting a signal representing the fact that the height of the sheet stack S reaches the predetermined level when the sensor lever 106 is rotated by a predetermined angle.
Incidentally, the sensor lever 106 is gradually rocked upwardly as the sheets S are successively stacked on the tray 103, and, when the height of the sheet stack S reaches the predetermined level, the sensor lever reaches the predetermined angle. With this arrangement, the sheet level detection sensor 105 can detect the fact that the height of the sheet stack S reaches the predetermined level.
When the fact that the height of the sheet stack S reaches the predetermined level is detected by the sheet level detection sensor 105 in this way, a tray lift/lower motor provided on the tray shifting stage 102 is driven, so that the tray 103 is lowered to substantially a position where a first sheet is discharged on the tray. As a result, it is possible to prevent any sheet from jamming at a sheet discharge opening 50 and to increase a maximum stackable amount of sheets on the tray 103.
Recently, there have been proposed sheet stacking apparatuses each having a plurality of trays. In such sheet stacking apparatuses, for example, when the sheets are successively received one by one in trays from an uppermost one to a lowermost one, after a maximum allowable sheets are stacked on a certain tray, it is necessary that this tray is shifted above the discharge opening and a next (lower) tray is shifted below the discharge opening.
However, when the tray on which the sheets were fully stacked is shifted, as is in the conventional case, if the conventional sheet level detection sensor is provided, since the sensor lever interferes with the movement of the tray, there must be provided for retarding the sensor lever to a position where the sensor lever does not interfere with the movement of the tray whenever the tray is lifted, thereby making the apparatus complicated.
Further, since the conventional sheet level detection sensor cannot detect whether the sheets are stacked on the tray or not, for example, there arises a problem that a different kind of sheet is stacked on the tray on which a certain kind of sheets were already stacked.